An internal combustion engine is typically operated under conditions in which there is insufficient oxygen or heat to oxidize all of the hydrocarbons present in the fuel being burned. By such a burning operation, there are developed exhaust gases which contain unburned hydrocarbons.
At present, it is proposed in the United States that motor vehicles be required to comply with emissions standards which progressively are becoming more stringent. Included within those proposals are requirements that hydrocarbon emissions be decreased significantly.
A number of methods have been developed and patented to oxidize the hydrocarbons in exhaust gases before those gases are exhausted into the atmosphere. However, none of those patents suggest the method or apparatus disclosed and claimed in this specification. Several of those patents are briefly reviewed here to show the state of the art.
U.S. Pat. No. 2,942,932 issued on Jun. 28, 1960 to Elliott for a "Process of Oxidizing Carbon Monoxide And Hydrocarbon In Exhaust Gases." The process disclosed comprises passing the exhaust gases during initial operation and warm up of the engine first through an adsorption zone in which hydrocarbons are adsorbed and then through an oxidation catalyst. As the engine warms up, hotter exhaust gases desorb the hydrocarbons from the adsorption zone while simultaneously bringing the oxidation catalyst to an effective operating temperature. After the exhaust gas reaches full operating temperature, the adsorption zone is in a regenerated condition and ready for a subsequent cold start-up. Means may also be provided for discontinuing the passage of hot exhaust gases through the adsorption zone after the oxidation catalyst has reached full operating temperature.
This method leaves certain problems unsolved. First, because the temperature at which the hydrocarbons will be desorbed is lower than the effective operating temperature of the oxidation catalyst, hydrocarbons will be desorbed from the adsorption zone before the oxidation catalyst is heated to an operational temperature. As a result, hydrocarbons desorbed from the adsorption zone will pass through the non-operational oxidation catalyst and be exhausted into the atmosphere. Second, because the adsorption zone precedes the oxidation catalyst, the exhaust gases used to desorb hydrocarbons from the adsorption zone will contain hydrocarbons. As a result, the adsorption zone will never be completely desorbed of hydrocarbons and the adsorption capacity of the adsorption zone during the next cold start of the engine will be reduced. Furthermore, the oxidation catalyst will be subjected to gases containing a larger quantity of hydrocarbons than normal--both those normally contained in the exhaust gases and those desorbed from the adsorption zone. If the hydrocarbon concentrations exceed the capacity of the oxidation catalyst, excess hydrocarbons will be exhausted into the atmosphere.
U.S. Pat. No. 3,150,192 issued on Sep. 29, 1964 to Ashley for a "Method Of Purifying Exhaust Gases Of Internal Combustion Engines." This patent discloses a method for oxidizing hydrocarbons from exhaust gases which comprises passing the exhaust gases sequentially through primary and secondary catalyst beds. During initial operation of the engine, hydrocarbons are adsorbed by the primary catalyst bed while auxiliary heat is applied to the secondary catalyst bed to rapidly raise the secondary catalyst bed to its oxidation temperature. The continuing passage of exhaust gases gradually raises the temperature of the primary bed catalyst to a point where the adsorbed hydrocarbons are desorbed and passed through the secondary catalyst bed which has been raised to its oxidation temperature.
While this method does provide for heating of the secondary catalyst bed to ensure that hydrocarbons later desorbed from the primary catalyst bed by the hot exhaust gases will be oxidized, this method still relies on hydrocarbon-laden exhaust gases to desorb the primary catalyst bed. Similar to the process claimed in the Elliott patent, this is not an efficient method of desorption and may lead to an excess discharge of hydrocarbons into the atmosphere.
U.S. Pat. No. 3,674,441 issued on Nov. 9, 1970 to Cole for an "Exhaust Emission Control." This patent shows a device for oxidizing hydrocarbons which comprises passing exhaust gases sequentially through a catalytic converter and a bed of storage material which retains hydrocarbons during warm up. After the catalytic converter is warmed up, the hydrocarbons retained in the storage bed are purged and recirculated back through the catalytic converter through the use of outside air.
This method avoids the problems posed by the patents already discussed by using outside air to desorb the adsorption storage material instead of exhaust gases. However, cooler outside air is not as efficient and requires much more time to desorb hydrocarbons as opposed to using hot exhaust gases from which the hydrocarbons have been oxidized. Because of this, if an internal combustion engine using the Cole method were operated repeatedly for short periods of time, the hydrocarbon storage bed would never be completely regenerated by the cooler outside air. In such a case, the hydrocarbons adsorbed in the hydrocarbon storage bed could build up during each start up until the hydrocarbon storage bed was saturated. If that happened, hydrocarbons would not be adsorbed by the saturated hydrocarbon trap during the next cold start up, but would simply be exhausted into the atmosphere.
U.S. Pat. No. 3,699,683 issued on Oct. 24, 1972 to Tourtellotte et al. for an "Engine Exhaust Emission Control System." This patent shows a method for oxidizing hydrocarbons by passing the exhaust gases emitted during engine start up first through an oxidizing catalyst bed and then through a hydrocarbon adsorbent bed which adsorbs hydrocarbons. When the engine exhaust gases are hot, most of the exhaust gases emitted from the oxidizing catalyst bed are then discharged directly into the atmosphere while the hydrocarbon adsorbent bed is regenerated by passing a small stream of hot exhaust gas through the hydrocarbon bed and recycling it to the engine.
The method disclosed in Tourtellotte et al. avoids the problem of using hydrocarbon laden exhaust gas to desorb hydrocarbons from the adsorbent bed by using exhaust gases from which hydrocarbons have already been oxidized in the catalyst bed to desorb hydrocarbons. However, this method does not account for the fact that during certain speed-load conditions, especially while the engine speed is changing in response to varying loads, the oxidation of hydrocarbons within the engine is much less efficient. During such conditions, the exhaust gases being oxidized in the catalyst bed contain a much higher percentage of hydrocarbons than would exist if the engine were running at steady state, that is, at a steady speed under steady load conditions and within a speed range where hydrocarbon oxidation within the engine is maximized. If the hydrocarbon bed is desorbed during periods when the engine is not running at steady state, then the desorbed hydrocarbons would be recycled back to the engine. Under those conditions, the percentage of unburned hydrocarbons in the exhaust gases exiting the engine and running through the catalyst bed would be even higher than normal. As a result, the catalyst bed would be subjected to exhaust gases abnormally rich in hydrocarbons. That could result in a loss of oxidation efficiency and the exhaust of gases into the atmosphere containing unburned hydrocarbons. Furthermore, if the hydrocarbon bed is desorbed during periods when the engine is not running at steady state, then the desorption gases will never be as hydrocarbon free as possible and the hydrocarbon bed will not be regenerated as quickly, as efficiently or as completely as would otherwise be possible.
Despite these and other teachings, there remains a need for a way to oxidize hydrocarbons efficiently at all stages of engine operation, and at the same time to reduce the exposure of the hydrocarbon adsorption materials to the high temperature gases needed to efficiently desorb them.